Power assistance device for an ultrasonic vibration dental handpiece

ABSTRACT

A power assistance device for an ultrasonic dental handpiece ( 5 ) includes a working circuit with a parallel impedance (Ls) between the output terminals (S 1 ,S 2 ) and a control circuit with a current transformer (T 2 ), the primary winding ( 7 ) thereof is serially arranged in the working circuit and the secondary winding ( 11 ) thereof forms an RLC circuit in conjunction with a capacitor ( 13 ) and a resistor ( 15 ) associated therewith. The voltage of the circuit at the terminals of the resistor ( 15 ) is transmitted to the input of a power supply ( 1 ). The control circuit enables variations in the value of the capacitor ( 13 ) and/or the value of the self-inductance coil of the secondary winding ( 11 ) of the transformer (T 2 ).

BACKGROUND OF THE INVENTION

The present invention relates to an electronic servo-control device fordental handpiece, of the type in which the vibration of a tool isobtained by means of a piezoelectric transducer.

DESCRIPTION OF THE RELATED ART

It is known that a piezoelectric transducer generating ultrasoundvibrations is, where possible, used in resonance so as to obtain maximumamplitudes and power of the vibrations. When such a transducer, to whicha tool is mechanically coupled, comes into contact during a work phasewith tissues of different natures, i.e. hard tissues, soft tissues, withor without the presence of a liquid, its resonant circuit evolves duringthe work. It is known that, in such a handpiece, the speed of vibrationof the transducer is a direct function of the electric current whichcirculates therein and that the effort necessary for this vibration is adirect function of the supply voltage at the terminals of saidtransducer. It will be understood that, if it is desired that ahandpiece operates with optimum yield, the vibrations of the transducermust correspond to the series resonance of this handpiece and, duringwork, the operational conditions must vary so as to remain in resonance.

SUMMARY OF THE INVENTION

According to the invention, the frequency will be tracked by observingthe phase-shift which exists between the voltage and the currentsupplied and by electrically compensating the intrinsic capacity of thetransducer. Such an electric circuit is translated in series resonanceby a low impedance and a zero phase-shift.

The present invention thus has for its object to propose such a devicefor servo-control of the piezoelectric transducer of a vibrationgenerator for dental handpiece, adapted to operate permanently at seriesresonance frequency, whatever the nature of the tissues on which thetool with which this handpiece is equipped, operates.

The present invention thus relates to a device for servo-control of adental handpiece activated by an ultrasound generator, comprising supplymeans of given frequency, characterized in that:

it comprises two circuits, namely a work circuit to whose terminals theultrasound generator is connected, and a control circuit,

the work circuit comprises an inductance in parallel between its outputterminals,

the supply is adapted to deliver at the output a voltage in phase with avoltage which is delivered thereto on its input,

the control circuit is constituted by an intensity transformer whoseprimary is arranged in series in the work circuit and whose secondaryforms, with a capacitor and a resistor associated therewith, an RLCcircuit of which the voltage at the terminals of the resistor is sent tothe input of said supply,

the control circuit comprises means for varying the value of thecapacitor and/or that of the self-induction coil of the secondary of theintensity transformer.

The secondary of the intensity transformer preferably comprises a coremobile inside its winding adapted to vary its inductance.

In a preferred embodiment, the supply means will be connected to thework circuit via a voltage transformer of which the inductances of theprimary and of the secondary will be high.

In an interesting form of embodiment of the invention, the inductancearranged between the output terminals of the work circuit will be suchthat, with the intrinsic capacitance of the handpiece and the internalresistance thereof, an RLC circuit close to the resonance is formed.

BRIEF DESCRIPTION OF THE DRAWINGS

A form of embodiment of the present invention will be describedhereinafter by way of non-limiting example, with reference to theaccompanying drawings, in which:

FIG. 1 schematically shows a frequency tracking device according to theinvention.

FIG. 2 schematically shows the phase-shifts between current andintensity in a circuit of the type shown in FIG. 1.

FIG. 3 is a curve representing the variation of the phase-shift betweencurrent and voltage in a circuit according to the invention as afunction of a multiple of the frequency.

FIG. 4 is a curve representing the respective variations as a functionof the frequency, of the power supplied to a specific handpiece and ofthe corresponding phase-shift between current and intensity.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The frequency tracking oscillator shown in FIG. 1 is essentiallyconstituted by a supply 1 capable of generating between its two outputterminals A and B a voltage V_(P) which supplies the primary 3 of avoltage transformer T₁. One of the terminals C of the secondary 4 ofthis transformer is connected to an output S1 of the circuit to which aninput E₁ of a handpiece 5 is connected. The other terminal D of thissame secondary 4 is connected to the other output S2 of the circuit withthe interposition of the primary 7 of a current transformer T₂. Thesecond input E2 of the handpiece 5 is connected to the terminal S2. Aninductance 9 of value L_(s) is arranged in parallel between the inputterminals E₁ and E₂ of the handpiece 5.

As is shown in FIG. 1, the secondary 11 of the intensity transformer T₂is arranged in series with a capacitor 13 of value C₂ and a resistor 15of value R₂, the latter representing the parasitic resistors of the RLCcircuit thus formed.

The terminals G and H of the resistor 15 are connected to inputterminals IJ of the supply 1.

There are thus two circuits, namely a work circuit which controls thehandpiece 5 and a control circuit constituted by the RLC circuit.

The supply 1 is constituted so that the voltage V_(P) produced on itsoutput terminals A, B is in phase with the voltage V_(r) existingbetween its input terminals I and J.

Under these conditions, as schematically shown in FIG. 2, for theoscillator constituted by the self-induction coil 11, the capacitor 13and the resistor 15 to enter in oscillation, the signal of voltage V_(r)collected at the terminals of the resistor R₂ must be in phase withV_(s), which condition is met if φ2=−φ1. In effect, φ2 and φ1 representthe phase-shift between voltage and intensity respectively in theoscillating control RLC circuit and in the work circuit controlling thevibrations of the handpiece 5.

If the voltage V_(r) existing between the input terminals I and J of thesupply 1 is expressed as a function of the current I₁ circulating in theprimary 7 of the transformer T2, it will be noted that the current I₁ isdelayed by φ1 with respect to voltage V_(s) (or to voltage V_(P)) andthat the voltage V_(r) is in phase with the current I₂.

If the equations of the transformer are taken into account, thefollowing will be obtained by using the complex mathematicalnotification:

V ₁ =Z ₁ I ₁ +jmωI ₂ with Z ₁ =jL ₁ω  (1)

0=Z ₂ I ₂ +jmωI ₁ with Z ₂ =R ₂ +j(L ₂ω−1/C ₂ω)  (2)

m representing the coefficient of mutual inductance of one of thewindings of the transformer on the other winding.

The transformer T₂ being an intensity transformer, it is possible, inknown manner, to disregard the influence of the secondary winding on theprimary winding so that the expression jmωI₂=0 and the value of I₁ isdrawn from equation (1), viz.:

I ₁ =V ₁ /jL ₁ ω=−jV ₁ /L ₁ω

By transferring this value in equation (2), the expression of thecurrent I₁ in the work circuit as a function of the current I₂ in theRLC circuit is obtained, viz.:

I ₁=1/mω(1/C _(1ω) −L _(2ω) +jR ₂)I ₂

Under these conditions, the phase-shift of the current I₂ with respectto current I₁ will be:

tgφ ₂ =R ₂/ω/(1/C ₂ ω−L ₂ω)=R ₂ C ₂ω/1−L ₂ C ₂ω²  (3)

Under these conditions, as mentioned hereinbefore, there will beoscillation if φ₂=−φ₁ or t_(g)φ₂=−t_(g)φ₁, viz. from the equation (3):

R ₂ C ₂ω/1−L ₂ C ₂ω² =−t _(g)φ₁  (4)

FIG. 3 shows the variation of the value of t_(g)φ₁ as a function of thevalue of ω which represents the vibration frequency, to within the valueof 2π(ω=2πN).

It will be noted that, without handpiece, the load of the oscillator inthe work circuit is reduced to the value of the inductance Ls arrangedin parallel between the output terminals S₁ and S₂ of the circuit.Furthermore, if R_(s) designates the internal resistance of theoscillator, the phase-shift of the current I_(l) with respect to V_(s)is expressed by the expression:

t _(gφ) =L _(s) /R _(s)

The condition of oscillation tgφ₂=−tgφ₁ then becomes:

R ₂ C ₂ω/(1−L ₂ C ₂ω²)=−L _(s)ω_(s) /R _(s)

 or ω²=(L _(s) +R _(s) R ₂ C ₂)/(L _(s) L ₂ C ₂)  (5)

By playing on the values of L₂ of the winding of the secondary 11 of thetransformer T₂ and/or the value C₂ of the capacitor 13, the frequency ofthe oscillator may be adjusted off-load so that the synchronizationcurve shown in FIG. 3 is modified.

The secondary 11 may include a core 19, the core 19 being mobile withinthe secondary 11 to vary to its inductance L₂.

In practice, R₂ represents the parasitic resistances of the circuit andC₂ will be conserved constant.

For each apparatus of a given series, it will then suffice to vary thevalue L₂ of the secondary 11 of the transformer T₂ until the voltage T₁is in phase with the current I₁ circulating in the circuit.

The apparatus will then be calibrated and the oscillator will “lock” onthe inductive delay load L_(s).

Furthermore, as shown in FIG. 4, a curve is available, which representsthe variation of the power at the terminals E1, E2 of the handpiece 5,as well as the value of the phase-shift between current and intensity atthe terminals thereof. Each type of handpiece 5 provided with adetermined tool will thus have a curve of this type.

In the example of FIG. 4, it will be observed that the power is maximumand the phase-shift is zero for a frequency of around 30 kHz. This valueplotted at point X in the diagram of FIG. 3 shows that the adjustment ofthe RLC circuit is correct since the value of tgφ₁ for this frequency isclose to 0.

It is, of course, known that, during operation of the handpiece, thevalue of the frequency for which a maximum vibration with zerophase-shift is obtained, varies as a function, on the one hand, of thephysical nature of the handpiece but also as a function of the surfacestate of the material to be treated. For a handpiece and a given tool,two extreme frequences N₁ and N₂ will therefore be obtained,corresponding to the tool working on soft tissues and harder elements,to which values X₁ and X₂ of ω will correspond, as shown in FIG. 3.

It has been observed that, in general, the frequency N lay at about 30kHz. Under these conditions, an off-load adjustment of each circuitproduced will be proceeded with (by adjusting the value of L₂ forexample) so that, during work, points X₁ and X₂ indeed lie within zonesfor which tg₁ is close to zero, as shown in FIG. 3.

The variation of the inductance L₂ may in particular be obtained bydisplacing a core at the centre of the self-induction coil 11.

What is claimed is:
 1. A device for servo-control of a dental handpiece,comprising: a supply means (1) with supply output terminals (A, B) andsupply input terminals (I, J), the supply input terminals supplied witha supply input voltage (Vr); a work circuit supplied by the supply meansand with output terminals (S1, S2) connected to input terminals (E1, E2)of a dental handpiece (5) and with a first inductance (Ls) connected inparallel between the output terminals of the work circuit; and a controlcircuit comprising an intensity transformer (T2) with a primary (7)arranged in series with the work circuit and a secondary (11) in serieswith a capacitor (13) and a resistor (15), the secondary in series withthe capacitor and the resistor forming an RLC circuit, terminals of theresistor being connected to the supply input terminals of the supplymeans so that a voltage at the terminals of the resistor is the supplyinput voltage, wherein, the supply means is adapted to deliver, at thesupply output terminals, a supply output voltage (Vs) in phase with thesupply input voltage applied at the supply input terminals, and one ofthe capacitor and a self-induction coil of the secondary of theintensity transformer are to providing one of a variable capacitance anda variable inductance.
 2. The device of claim 1, wherein the secondaryof the intensity transformer comprises a core (19), the core beingmobile within a winding of the secondary to vary an inductance of thesecondary.
 3. The device of claim 1, wherein the work circuit isconnected to the supply output terminals via a voltage transformer (T1).4. The device of claim 1, wherein the first inductance is positionedsuch that an inductance of the first inductance and an intrinsiccapacitance of the handpiece and an internal resistance of the handpieceform an approximately resonant RLC circuit.
 5. A servo-control devicefor a dental handpiece, comprising: a supply (1) with supply outputterminals (A, B) and supply input terminals (I, J), the supply inputterminals supplied with a supply input voltage (Vr); a work circuitoperatively supplied by the supply and with output terminals (S1, S2)connected to input terminals (E1, E2) of a dental handpiece (5) and witha first inductance (Ls) connected in parallel between the outputterminals of the work circuit; and a control circuit comprising anintensity transformer (T2) with a primary (7) arranged in series withthe work circuit and a secondary (11) in series with a capacitor (13)and a resistor (15), the secondary in series with the capacitor and theresistor forming a first RLC circuit, terminals of the resistor beingconnected to the supply input terminals of the supply so that a voltageat the terminals of the resistor is connected as the supply inputvoltage at the supply input terminals, wherein, the supply is adapted todeliver, at the supply output terminals, a supply output voltage (Vs) inphase with the supply input voltage applied at the supply inputterminals, and one of the capacitor and a self-induction coil of thesecondary of the intensity transformer are variable to providing one ofa variable capacitance and a variable inductance.
 6. The device of claim5, wherein the secondary of the intensity transformer comprises a core(19) mobile within a winding of the secondary.
 7. The device of claim 6,wherein a voltage transformer (T1) connects the work circuit to thesupply output terminals.
 8. The device of claim 6, wherein the firstinductance is positioned such that an inductance of the first inductanceand an intrinsic capacitance of the handpiece and an internal resistanceof the handpiece form a second RLC circuit.
 9. The device of claim 8,wherein the second RLC circuit is a resonant RLC circuit.
 10. The deviceof claim 5, wherein a voltage transformer (T1) connects the work circuitto the supply output terminals.
 11. The device of claim 5, wherein thefirst inductance is positioned such that an inductance of the firstinductance and an intrinsic capacitance of the handpiece and an internalresistance of the handpiece form a second RLC circuit.
 12. The device ofclaim 11, wherein the second RLC circuit is a resonant RLC circuit. 13.A servo-control device, comprising: a supply (1) with supply outputterminals (A, B) and supply input terminals (I, J), the supply inputterminals supplied with a supply input voltage (Vr); a work circuitoperatively supplied by the supply and with output terminals (S1, S2)connected to input terminals (E1, E2) of a load (5) and with a firstinductance (Ls) connected in parallel between the output terminals ofthe work circuit; and a control circuit comprising an intensitytransformer (T2) with a primary (7) arranged in series with the workcircuit and a secondary (11) in series with a capacitor (13) and aresistor (15), the secondary in series with the capacitor and theresistor forming a first RLC circuit, terminals of the resistor beingconnected to the supply input terminals of the supply so that a voltageat the terminals of the resistor is connected as the supply inputvoltage at the supply input terminals, wherein, the supply is adapted todeliver, at the supply output terminals, a supply output voltage (Vs) inphase with the supply input voltage applied at the supply inputterminals.
 14. The device of claim 13, wherein, one of the capacitor anda self-induction coil of the secondary of the intensity transformer arevariable to providing one of a variable capacitance and a variableinductance.
 15. The device of claim 14, wherein the secondary of theintensity transformer comprises a core (19) mobile within a winding ofthe secondary.
 16. The device of claim 13, wherein a voltage transformer(T1) connects the work circuit to the supply output terminals.
 17. Thedevice of claim 13, wherein the first inductance is positioned such thatan inductance of the first inductance and an intrinsic capacitance ofthe load and an internal resistance of the load form a resonant, secondRLC circuit.